Generally, a power shift transmission includes a number of gear elements coupling the input and output shafts, and a related number of clutches which are selectively engagable to activate gear elements for establishing a desired speed ratio between the input and output shafts. The clutch may be of the band type or the disk type.
For example, the input shaft may be connected to the engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle drive. Shifting from one gear ratio to another involves releasing or disengaging the off-going clutches associated with the current gear ratio and applying or engaging the on-coming clutches associated with the desired gear ratio.
To improve the shifting of the transmission, electronic control technology is commonly used, wherein a direct interface between an electronic control unit and the clutches is provided via a plurality of clutch fill control elements such as solenoid valves. The solenoid valves are modulated to control both the on-coming and off-going clutch pressures in response to input pulses from the electronic control unit.
To provide for precise timing of the shifting, it is desirous to determine the fill time of an on-coming clutch. The fill time is the time required to fill an on-coming clutch cavity with fluid.
Various clutch control systems and strategies for filling on-coming clutches are known. Reference, for instance, Brandon et al U.S. Pat. No. 5,449,329 issued Sep. 12, 1995 to Deere & Company; Holbrook et al U.S. Pat. No. 5,468,198 issued Nov. 21, 1995 to Chrysler Corporation; Creger et al U.S. Pat. No. 5,551,930 issued Sep. 3, 1996 to Caterpillar, Inc.; and Mitchell et al U.S. Pat. No. 5,580,332 issued Dec. 3, 1996 to Caterpillar, Inc.
Commonly, fill strategies involve first pre-filling the clutch at a high fill pressure to a level just below that required to fully engage the clutch, ramping down the pressure to a lower pressure, holding the lower pressure, then gradually increasing the pressure again to the high level to fully engage the clutch. The volume of the clutch cavity is a known constant. However, the fluid flow rate to the clutch has been found to vary with the hydraulic system pump speed, which is a function of the engine speed. Since engine speed varies widely, typically at least two predetermined nominal pre-fill times are used, including a longer nominal pre-fill time for low speed ranges, and a shorter nominal pre-fill time for higher speed ranges. At the higher and lower engine speeds for which the nominal pre-fill times were selected, shifting is generally smooth and satisfactory. However, when shifting at engine speeds between the higher and lower ranges for which the nominal pre-fill times were selected, the shorter pre-fill time is used so as to not fully fill the clutch during the pre-fill period. In many cases at these intermediate engine speeds, particularly when downshifting under coasting conditions and the like, the shift is rougher and/or more jerky than desired. This has been found to be due to inadequate pre-filling of the clutch, as a result of a low fluid flow rate thereto.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.